1. Field of the Invention
The present invention relates to a method of protecting data in a data transmission system and is an improvement on known methods of protecting of telecommunications traffic in a transmission network, particularly but not exclusively in a network which is based on the SDH (synchronous digital hierarchy).
The invention is described in terms of digital traffic at the 2,048 kbit/s ("2 Mbit/s") rate, but can apply to any data or other traffic of any modulation scheme or information rate, provided that it has attributes similar to those in the present invention.
2. Description of the Related Art
One necessary attribute is that the data has some associated information with which the quality of the data can be checked, and which travels with the data. In the extreme case, total loss of the data would be taken as a failure of quality, but in more normal circumstances, corruption of parity or other type of digital check sequence would be an example of how quality can be measured, Signals based on the SDH contain a type of parity data (BIP, or "byte interleaved parity") which is used in this way.
The equipment in which the invention is to be used is an add-drop multiplexer (also known as an ADM or drop-insert multiplexer). An ADM provides low cost access to all or part of the traffic passing along a telecommunications bearer, such as an optical fibre. The traffic passing through does so via "line ports", and the accessed traffic passes via `tributary ports`. It is conventional in transmission networks to provide protection against bearer failure by provision of one or more alternative bearers, which are selected either automatically or under operator control. In the former case the control is based on failure or on a quality check applied to the received signals. In the latter case it is based conventionally on reports of such conditions, passed to a central control point for operator action, The traffic in the return direction may be left undisturbed or may be switched between paths in sympathy with the direction of traffic which has experienced the fault, in order to simplify maintenance of duplex traffic equipment.
The main advantages of automatic control are that protection is generally faster, and operating costs are lower. The chief disadvantage, however, is that flexibility of rerouting is generally limited. If the flexibility automatically to choose from many alternative routes is provided, then the response is usually slow, because of the limitations of computer processing power, with the result that disturbance continues for longer after a break before restoration occurs.
The simplest example of alternative routing is selection of a standby path which may be shared by two or more working paths between the same end points. Another is the use of a ring to provide two paths, generally over diverse routes, and either path is chosen as necessary. In most cases, as in the examples just given, them is a similarity or a symmetry, or other simplifying characteristic, between the alternative paths, so that the design or operation of the control system is not too complex, and therefore, operation can be fast.
Even for more complex networks, the rules for selection of an alternative route can be made sufficiently simple for fast automatic rerouting to be provided, generally by the operator pre-assigning a sub-set of the possible mutes, from which the control system selects a suitable one. Again, however, the routes or paths have some similarity or symmetry, such as being at the same data rate.
The arrival of SDH has coincided with an increase in the use of rings, and several protection schemes for use in rings are already known. To date, known schemes restrict the protection to within the ring, or extend it outside the ring in a generally symmetrical fashion.
A situation arises in which a single network node (the "dependant" node) is required to have a very high security of connection to a ring. Although traffic within the ring can be protected by known means such as are referred to above, traffic to and from the ring can normally be protected automatically only by replication of the connections between it and one equipment on the ring. Any other arrangement would normally result in the control difficulties mentioned above, and automatic protection is likely to be slow.
If speed response is not a constraint then ideally, there would be connections between the node and two or more equipments on the ring, so that if one were to fail, traffic to and from the ring would survive. Such an equipment is referred to here as a parent, and the arrangement to be described is known as dual parenting. The purpose of this invention is to provide a very fast protection means in this case.